Fierce battles between Syrian regime forces and the Islamic State jihadist group in eastern Syria have left 34 fighters dead in 24 hours, a monitor said Friday.

The Syrian Observatory for Human Rights said 19 pro-government forces and 15 IS extremists had been killed since clashes began late Wednesday in the city of Deir Ezzor and around its nearby military airport.

The head of aerial defence at the airport, one of the few areas left in regime hands in Deir Ezzor province, was killed in the fighting, the Observatory said.

Four government soldiers were beheaded by IS Thursday as the jihadists seized a key checkpoint in the city near the air base.

"An IS suicide bomber detonated himself by the checkpoint, which IS then seized," said Observatory director Rami Abdel Rahman.

"Seizing the checkpoint gets them closer to the military airport."

Abdel Rahman said fighting continued into Friday morning with both sides shelling positions on the outskirts of Deir Ezzor city.

Local activist Mohammad al-Khleif confirmed IS had gained control of the Jamyan checkpoint in the city's southeast.

IS already controls most of Deir Ezzor province and roughly half its capital, he told AFP.

If its offensive succeeds, Deir Ezzor would be the second provincial capital to fall to the group, after it named the northern city of Raqa the capital of its 'caliphate'.

In northern Syria, at least six civilians were killed Thursday in attacks in Aleppo city, once Syria's commercial hub but now divided between government control in the west and rebels in the east.

The Britain-based Observatory said a woman and three children died after rebel rocket attacks on the government-held Ashrafiyeh neighbourhood.

A father and his young son were killed by snipers in another regime-held area.

In Aleppo province, a fierce attack by IS on the regime's Kweyris military airport left seven pro-government forces dead.

According to the Observatory, IS has surrounded the airport but are coming under fire from regime war planes.

South-east of Syria's capital, an explosion in the Billi military airport killed 13 regime soldiers including the base's head, the Observatory said.

"The blast happened inside a plane within the base, but the causes are still unclear," Abdel Rahman said.

Billi air base lies 40 kilometres (25 miles) south-east of Damascus along a main highway that leads to the regime-controlled city of Sweida.